Taken from: https://salsa.debian.org/glibc-team/glibc/-/blob/9ce19663f00176f30d6eab29fd14db3e7cd53dcf/debian/patches/hurd-i386/local-clock_gettime_MONOTONIC.diff

Use the realtime clock for the monotonic clock. This is of course not a proper
implementation (which is being done in Mach), but will permit to fix at least
the iceweasel stack.

vlc however doesn't build when _POSIX_CLOCK_SELECTION is enabled but
_POSIX_TIMERS is not, and they refuse to fix that (see #765578), so disable the
former.

---
 sysdeps/mach/hurd/bits/posix_opt.h |    2 +-
 sysdeps/unix/clock_gettime.c       |    1 +
 2 files changed, 2 insertions(+), 1 deletion(-)
Index: glibc-2.27/sysdeps/mach/clock_gettime.c
===================================================================
--- glibc-2.27.orig/sysdeps/mach/clock_gettime.c
+++ glibc-2.27/sysdeps/mach/clock_gettime.c
@@ -31,6 +31,10 @@ __clock_gettime (clockid_t clock_id, str
   switch (clock_id) {
 
     case CLOCK_REALTIME:
+    case CLOCK_MONOTONIC:
+    case CLOCK_MONOTONIC_RAW:
+    case CLOCK_REALTIME_COARSE:
+    case CLOCK_MONOTONIC_COARSE:
       {
 	/* __host_get_time can only fail if passed an invalid host_t.
 	   __mach_host_self could theoretically fail (producing an
Index: glibc-2.27/rt/timer_create.c
===================================================================
--- glibc-2.27.orig/rt/timer_create.c
+++ glibc-2.27/rt/timer_create.c
@@ -48,7 +48,7 @@ timer_create (clockid_t clock_id, struct
       return -1;
     }
 
-  if (clock_id != CLOCK_REALTIME)
+  if (clock_id != CLOCK_REALTIME && clock_id != CLOCK_MONOTONIC && clock_id != CLOCK_MONOTONIC_RAW && clock_id != CLOCK_REALTIME_COARSE && clock_id != CLOCK_MONOTONIC_COARSE)
     {
       __set_errno (EINVAL);
       return -1;
Index: glibc-2.27/sysdeps/mach/hurd/bits/posix_opt.h
===================================================================
--- glibc-2.27.orig/sysdeps/mach/hurd/bits/posix_opt.h
+++ glibc-2.27/sysdeps/mach/hurd/bits/posix_opt.h
@@ -163,10 +163,10 @@
 #define _POSIX_THREAD_PROCESS_SHARED	-1
 
 /* The monotonic clock might be available.  */
-#define _POSIX_MONOTONIC_CLOCK	0
+#define _POSIX_MONOTONIC_CLOCK	200809L
 
-/* The clock selection interfaces are available.  */
-#define _POSIX_CLOCK_SELECTION	200809L
+/* The clock selection interfaces are not really available yet.  */
+#define _POSIX_CLOCK_SELECTION	-1
 
 /* Advisory information interfaces could be available in future.  */
 #define _POSIX_ADVISORY_INFO	0
Index: glibc-upstream/sysdeps/posix/clock_getres.c
===================================================================
--- glibc-upstream.orig/sysdeps/posix/clock_getres.c
+++ glibc-upstream/sysdeps/posix/clock_getres.c
@@ -52,6 +52,10 @@ __clock_getres (clockid_t clock_id, stru
   switch (clock_id)
     {
     case CLOCK_REALTIME:
+    case CLOCK_MONOTONIC:
+    case CLOCK_MONOTONIC_RAW:
+    case CLOCK_REALTIME_COARSE:
+    case CLOCK_MONOTONIC_COARSE:
       retval = realtime_getres (res);
       break;
 
--- ./sysdeps/mach/clock_nanosleep.c.original	2020-07-21 00:31:35.226113142 +0200
+++ ./sysdeps/mach/clock_nanosleep.c	2020-07-21 00:31:49.026185761 +0200
@@ -62,7 +62,7 @@
 __clock_nanosleep (clockid_t clock_id, int flags, const struct timespec *req,
 		   struct timespec *rem)
 {
-  if (clock_id != CLOCK_REALTIME
+  if ((clock_id != CLOCK_REALTIME && clock_id != CLOCK_MONOTONIC && clock_id != CLOCK_MONOTONIC_RAW && clock_id != CLOCK_REALTIME_COARSE && clock_id != CLOCK_MONOTONIC_COARSE)
       || !valid_nanoseconds (req->tv_nsec)
       || (flags != 0 && flags != TIMER_ABSTIME))
     return EINVAL;
Index: glibc-2.32/hurd/hurdlock.c
===================================================================
--- glibc-2.32.orig/hurd/hurdlock.c
+++ glibc-2.32/hurd/hurdlock.c
@@ -47,7 +47,7 @@ int
 __lll_abstimed_wait (void *ptr, int val,
   const struct timespec *tsp, int flags, int clk)
 {
-  if (clk != CLOCK_REALTIME)
+  if (clk != CLOCK_REALTIME && clk != CLOCK_MONOTONIC)
     return EINVAL;
 
   int mlsec = compute_reltime (tsp, clk);
@@ -59,7 +59,7 @@ int
 __lll_abstimed_wait_intr (void *ptr, int val,
   const struct timespec *tsp, int flags, int clk)
 {
-  if (clk != CLOCK_REALTIME)
+  if (clk != CLOCK_REALTIME && clk != CLOCK_MONOTONIC)
     return EINVAL;
 
   int mlsec = compute_reltime (tsp, clk);
@@ -79,7 +79,7 @@ int
 __lll_abstimed_xwait (void *ptr, int lo, int hi,
   const struct timespec *tsp, int flags, int clk)
 {
-  if (clk != CLOCK_REALTIME)
+  if (clk != CLOCK_REALTIME && clk != CLOCK_MONOTONIC)
     return EINVAL;
 
   int mlsec = compute_reltime (tsp, clk);
@@ -91,7 +91,7 @@ int
 __lll_abstimed_lock (void *ptr,
   const struct timespec *tsp, int flags, int clk)
 {
-  if (clk != CLOCK_REALTIME)
+  if (clk != CLOCK_REALTIME && clk != CLOCK_MONOTONIC)
     return EINVAL;
 
   if (__lll_trylock (ptr) == 0)
@@ -177,7 +177,7 @@ __lll_robust_abstimed_lock (void *ptr,
   int wait_time = 25;
   unsigned int val;
 
-  if (clk != CLOCK_REALTIME)
+  if (clk != CLOCK_REALTIME && clk != CLOCK_MONOTONIC)
     return EINVAL;
 
   while (1)
